Off the coast of Yonaguni, a small island near Okinawa, divers work a submerged rock formation with straight edges, right angles, and stepped terraces that some call the ruins of a lost civilization and others call an unusual but entirely natural sandstone outcrop. The film, an episode of the History's Mysteries series, lays out why the site matters: conventional archaeology dates organized human civilization to roughly 5,000 years ago, but if the Yonaguni formation is man-made, it would have been carved and later submerged by rising seas at the end of the last ice age, some 10,000 years back. Underwater footage shows the terraces, ledges, and what look like carved steps up close, while researchers on both sides of the debate weigh in, mainstream geologists arguing for natural rock fracture patterns, independent investigators arguing for deliberate construction. No excavation resolves it here. What the film delivers instead is the case file itself, the footage, the competing readings of the same stone, and the open question of whether Japan's seafloor is hiding a chapter of human history nobody has written yet.